Many modern systems include air or liquid filters to ensure proper or reliable performance. For example, automobiles include air and fuel filters to remove dirt and other particulates from the air and fuel that are mixed and then ignited within their internal combustion engines. As a consequence of their proper operation, these filters collect particulates over time and increasingly restrict the flow of air or fuel into engines. Eventually, the filters become more restrictive than desirable to promote fuel efficiency or other performance criteria and require replacement.
To facilitate timely filter replacement, automobiles and other systems sometimes include filter-monitoring devices, which monitor pressure or vacuum levels that result from fluid flow through associated filters. These devices are calibrated to detect when particular pressure or vacuum conditions occur and to respond to such occurrences in particular ways. For example, some devices, referred to herein as dial-type flow restriction gauges (or indicators) provide a dial that rotates relative to a fixed scale in response to differential pressure, thereby providing a visible measurement of flow restriction through a filter.
The present inventors have recognized that commercially available dial-type flow-restriction gauges are not only more costly to make than desirable, but also prone to reliability and performance issues. For example, one known gauge provides a dial within a two-piece housing having a clear plastic upper portion and an opaque plastic lower portion. The clear plastic portion provides a view of the internal dial relative to a restriction scale printed on the exterior of the clear plastic. However, suitable clear plastic is not only more expensive (two-to-three times more), but more vulnerable to degrading reactions with fuel, oil, and other chemicals common to automobile engine environments than the opaque portion. This known indicator also uses a relatively expensive spiral spring to bias rotation of the dial and a separate O-ring to seal the housing.
Accordingly, the present inventors have recognized a need to improve conventional dial-type flow-restriction gauges.